The below-identified Patent document 1 discloses a hydraulic booster including an auxiliary hydraulic pressure source having a power pump and a pressure accumulator, and a pressure regulator including a spool valve and configured to adjust the hydraulic pressure supplied from the auxiliary hydraulic pressure source to a value corresponding to the operating amount of a brake operating member, and to introduce the thus adjusted hydraulic pressure into a boost chamber as an assisting force, in which an assisted force (sum of the brake operating force applied by a driver of the vehicle and the above assisting force) is applied to the piston of the master cylinder.
Hydraulic brake systems are now commercially available which includes a circulation type pressure control unit including an electronic control unit which performs anti-lock brake control (ABS) and/or electronic (vehicle) stability control (ESC).
The electronic control unit of the circulation type, pressure control unit receives information from various known sensors for detecting e.g. wheel speeds, the stroke of the brake operating member, brake hydraulic pressure, and the behavior of the vehicle. When the electronic control unit determines, based on the information received, that it is necessary to reduce pressure of at least one wheel cylinder, the electronic control unit closes a pressure increasing solenoid valve in the hydraulic line extending from the master cylinder to the wheel cylinder, and opens a pressure reducing solenoid valve in a hydraulic line extending from the wheel cylinder to a low-pressure fluid reservoir, thereby reducing the wheel cylinder pressure.
When the electronic control unit determines, thereafter, that it is necessary to reincrease the wheel cylinder pressure, the electronic control unit opens the pressure increasing solenoid valve, closes the pressure reducing solenoid valve, and activates a circulating power pump to draw brake fluid in the low-pressure fluid reservoir and return the thus drawn brake fluid into the hydraulic line extending from the master cylinder to the wheel cylinder.
Some of such hydraulic brake systems having the above-described circulation type pressure control unit include a shut-off valve provided in the hydraulic line extending from the master cylinder to each wheel cylinder at its point upstream of the point (return point) at which the brake fluid drawn by the circulating pump is returned into the hydraulic line, i.e. the point between the master cylinder and the return point, and others do not have such a shut-off valve. The shut-off valve is closed during e.g. ABS control.
In the arrangement without the shut-off valve, brake fluid drawn by the circulating pump flows back toward the master cylinder (this phenomenon is often called, and hereinafter referred to as “pump back flow”).